Communication services have experienced a fast growing demand, for example, generalization of information communication services, introduction of various multimedia services, and provision of high-quality services. To meet such a demand, various wireless communication techniques have been studied in various fields.
In next generation wireless communication systems, massive multimedia data can be transmitted at a high speed by using limited radio resources. To achieve this, spectral efficiency has to be maximized since a radio channel has limited bandwidth. In addition, inter-symbol interference and frequency selective fading, which occur during high-speed mobility, have to be overcome. Various techniques have been developed to maximize the spectral efficiency, and among them, an orthogonal frequency division multiplexing (OFDM) scheme and a multiple input multiple output (MIMO) scheme are most promising techniques.
The OFDM uses a plurality of orthogonal sub-carriers. Further, the OFDM uses orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). A transmitter transmits data after performing IFFT. A receiver restores original data by performing FFT on a received signal. The transmitter uses IFFT to combine the plurality of sub-carriers, and the receiver uses FFT to split the plurality of sub-carriers. According to the OFDM, complexity of the receiver can be reduced in a frequency selective fading environment of a broadband channel, and the spectral efficiency can be improved through selective scheduling in frequency domain by utilizing channel characteristics which are different from one subcarrier to another. An orthogonal frequency division multiple access (OFDMA) is an OFDM-based multiple access scheme. According to the OFDMA, radio resources can be more efficiently used by allocating different subcarriers to multi-users.
The MIMO scheme can be mainly used for two purposes. A first purpose is to increase diversity gain in order to reduce performance degradation resulted from a channel fading environment. A second purpose is to increase a data transmission rate. The MIMO scheme outperforms a single-input single-output (SISO) system using one transmit/receive antenna in that more data can be transmitted without having to increase the frequency band.
A MIMO channel is provided by multiple antennas and can be decomposed into independent channels. If the number of transmit antennas is Nt and the number of receive antennas is Nr, then the number of independent channels is Ni, where Ni≦min{Nr, Ni}. In this case, each independent channel can be referred to as a spatial layer. In general, a rank is defined as the number of non-zero eigen-values of a MIMO channel matrix and corresponds to the maximum number of independent channels.
In a MIMO system, a transmitter performs precoding in which a transmit signal is multiplied by a weight. The precoding is a scheme in which the transmit signal is pre-processed by using the weight before transmission. The weight is selected or calculated based on channel condition on which the transmit signal is transmitted. In general, in a frequency division duplex (FDD) system, a receiver determines a weight and transmits the weight to a transmitter, and the transmitter determines a actual weight, which is used in actual transmission, based on the received weight. In a time division duplex (TDD) system, the transmitter determines the weight by using a sounding channel.
In order for the receiver to post-process the precoded signal, the receiver has to know the weight. That is, the transmitter needs to send to the receiver information on which weight is used. In general, the weight is configured in a form of a vector or a matrix. Thus, if information on every elements of the vector or the matrix is transmitted without processing, the spectral efficiency may deteriorate due to a large overhead.
Accordingly, there is need for a method for efficiently transmitting weight information in a multiple antenna system.